Method for the detection of misfires in an internal combustion engine

ABSTRACT

According to methods known in prior art, a parameter which depends on the acceleration of the internal combustion engine is determined by means of a monitoring and analyzing system and is compared with a threshold value. According to the inventive method, the spread of said acceleration-dependent parameter is used for adjusting the threshold value to changes in the smoothness of running of the internal combustion engine.

CROSS REFERENCE TO RELATED APPLICATION

This application is the U.S. National Stage of International ApplicationNo. PCT/DE2003/003273, filed Oct. 1, 2003 and claims the benefitthereof. The International Application claims the benefits of GermanPatent applications No. 10254479.4 DE filed Nov. 21, 2002, all of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to a method for detecting misfires in aninternal combustion engine.

BACKGROUND OF THE INVENTION

Numerous methods for the detection of misfires in internal combustionengines are already known; see, for example, EP 0 708 234, EP 0 716 298and U.S. Pat. No. 5,056,360. Said methods make use of the physicaleffect that a cylinder in which a misfire occurs exhibits a smalleracceleration value than adjacent cylinders. In the methods known in theprior art, this physical effect is made use of in such a way that aparameter dependent on the acceleration of the internal combustionengine, such as, for example, an acceleration index or what is known asa cylinder segment time is continually determined by means of amonitoring and analysis method while the internal combustion engine isrunning. Said parameter is then compared with a threshold value. Thethreshold value is defined as a function of the operating point of theinternal combustion engine (e.g. as a function of the speed and load),and it is generally stored once during the calibration of the internalcombustion engine in the operation control device of the internalcombustion engine in the form of engine characteristic maps. Thus, ifthe parameter continually determined for the acceleration of theinternal combustion engine falls below this threshold value, it isinterpreted as the detection of a misfire in the cylinder in question.

A basic problem with these detection methods is that it is verydifficult in specific operating phases of the internal combustion engineto differentiate speed variations caused by misfires fromoperation-related speed variations. Operating phases occurring at highspeed and low load are particularly affected. At high speeds the timeintervals (segment times) to be measured become shorter and shorter,with the result that it is not possible to define a threshold value thathas a sufficiently large gap with respect to the continually determinedspeed-dependent parameter to allow error-free detection of misfires.

This also applies to operation of the internal combustion engine withnon-optimal operating parameters, as is necessary for example forheating catalytic converters. In order to accelerate the heatingprocess, the internal combustion engine is operated for example with anincreased quantity of air and fuel, but with very late firing.Consequently, the firing and combustion of the fuel partially take placedirectly in the catalytic converter and not in the cylinder. The resultis a very rapid increase in the exhaust temperature. As the internalcombustion engine is operated in this case at a very late firing anglerather than at its optimal firing angle, there is also an increase inthe uneven running of the internal combustion engine. This then leads toan increase in and a correspondingly large variance in theacceleration-dependent parameter, which makes misfire detectioncorrespondingly more difficult.

In the prior art numerous algorithms were developed in order to takeinterfering influences in misfire detection into account and also permitreliable misfire detection even under unfavorable operating conditionsof the internal combustion engine. Thus, for example, a switch can bemade from one threshold value to another when switching between certainoperating phases (catalytic converter heating or not). Numerous furtherrefinements of the algorithms for misfire detection are also known, bymeans of which there has been a large measure of success in detectingmisfires in relatively wide operating ranges of the internal combustionengine with sufficient reliability. However, this must generally beaccomplished at the expense of a relatively computing and storageoverhead in the operation control device of the internal combustionengine.

SUMMARY OF THE INVENTION

The object of the present invention is to specify a method for detectingmisfires in an internal combustion engine that allows, in the simplestmanner possible, reliable misfire detection even under unfavorableoperating conditions.

The method according to the invention is defined in the claims.

In the method according to the invention, the starting point is aconventional misfire detection method in which a parameter dependent onthe acceleration of the internal combustion engine is continuallydetermined by means of a predefined monitoring and analysis method whilethe internal combustion engine is running and compared with a thresholdvalue. The acceleration-dependent parameter is for example anacceleration index, a torque index, a segment time or a similarvariable, as known for example from the aforementioned publications. Inprinciple this parameter can be determined using any known monitoringand analysis method with a more or less complex algorithm, as islikewise known from the aforementioned publications.

The acceleration-dependent parameter is a reflection of the combustionefficiency, since it represents a measure for the torque contribution,generated by the combustion, of the individual cylinders. The varianceor cyclical distribution of this parameter therefore reflects the evenrunning of the internal combustion engine.

According to the invention, therefore, the variance of theacceleration-dependent parameter is determined and used to adjust thethreshold value to changes in the even running of the internalcombustion engine. In particular, the threshold value is increased ifthere is a reduction in the even running of the engine, and reduced ifthere is an increase in the even running of the engine.

In this way the threshold value for misfire detection can be continuallyand automatically adjusted to take account of changing operating states,thereby increasing the reliability of the misfire detection accordingly.

The variation range for the variance in the acceleration-dependentparameter can be specified in an arbitrary manner. A predefined timeinterval or a predefined number of power strokes is used as a variationrange, for example.

The method according to the invention increases the reliability andaccuracy of misfire detection across the entire operating range andduring the entire lifetime of the internal combustion engine. Reliablemisfire detection is provided in particular also during unfavorableoperating phases such as, for example, during the heating of thecatalytic converter. A further advantage of the method according to theinvention is that it can be used during the calibration of the internalcombustion engine, with the result that no separate specification of athreshold value for misfire detection is necessary. This simplifies thecalibration method. All this is achieved with minimal computing andstorage overhead, which means a corresponding reduction in the load onthe electronic operation control device. The invention therefore makes acontribution to the optimization of the operation of the internalcombustion engine with regard to fuel consumption and exhaust emissions.

Methods for controlling the even running of an internal combustionengine are already known in the prior art. With such methods, thecombustion in the individual cylinders is corrected by modification ofspecific operating parameters such as injected fuel quantity, firingmoment, etc. in such a way that the even running of the internalcombustion engine is increased. Reference may be made for example to DE197 41 965 C1. With this method the difference between actual value anddesired value of a characteristic process variable, in particular aprocess variable that is dependent on the rotational acceleration of theindividual cylinders, is used to correct the combustion in theindividual cylinders. Variations in rotational acceleration between theindividual cylinders are then compensated for by modification inparticular of the allocated fuel quantity for each individual cylinder.

In further embodiment of the invention it is thus provided that thevariance in the parameter dependent on the acceleration of the internalcombustion engine is used for checking the result of the even runningregulation process. If, for example, after the even running regulationhas been performed and the adjustment of the threshold value has beencompleted, misfires continue to occur in one or more cylinders, thechecking method according to the invention detects the combustion ofsaid cylinder or cylinders as defective. In this way the malfunctioningof the cylinder in question is thus confirmed or verified. A faultmessage can then be generated.

As the variance of the acceleration-dependent parameter is determined inany case with the method according to the invention for adjusting thethreshold value, the implementation of the described checking methodrequires only a slight additional overhead.

Further advantageous embodiments of the invention are derived from thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be explained in more detailwith reference to the drawing, in which:

FIG. 1 shows a diagram in which a parameter K (acceleration index) isplotted against the power strokes Z of an internal combustion engine;

FIG. 2 is a flowchart of a method for adjusting a threshold value formisfire detection;

FIG. 3 is a flowchart of a method for checking the result of an evenrunning regulation.

DETAILED DESCRIPTION OF THE INVENTION

As explained in the introduction, with the conventional misfiredetection methods a parameter dependent on the acceleration of theinternal combustion engine is continually determined by means of amonitoring and analysis method while the internal combustion engine isrunning and then compared with a predefined threshold value. In FIG. 1,a parameter K of this kind is plotted against the power strokes Z of aninternal combustion engine. In the exemplary embodiment shown, theparameter K is an acceleration index, as known for example from U.S.Pat. No. 5,056,360 cited at the beginning.

As the type of the monitoring and analysis method for determining theparameter K is not important in the present context and as, besides,monitoring and analysis methods of the said type are known, it will notbe dealt with in further detail at this juncture. It is sufficient topoint out that the acceleration index represents a measure for theacceleration of the crankshaft in a specific operating point of theinternal combustion engine. If the acceleration index falls below thethreshold value, this signifies that the cylinder in question has madeno or only an inadequate torque contribution in this operating point,which is generally to be attributed to a misfire.

However, other parameters dependent on the acceleration of the internalcombustion engine, such as, for example, the so-called segment times,could also be used instead of the acceleration index.

The segment times are the time intervals which the crankshaft requiresduring the power strokes of the individual cylinders for executingpredefined angular spans. Since the determination and evaluation ofsegment times is also known, there is no need to explore this topicfurther here.

The diagram shown in FIG. 1 shows the progression of the parameter K(acceleration index). On the left-hand side of the diagram the parameterK is shown during optimal operation of the internal combustion engine.As can be seen, the parameter K changes only slightly in this operatingrange. The straight line designated by S represents a suitable thresholdvalue for this operating range. If the parameter K drops below thethreshold value S into this operating range, this indicates a misfireVA.

As can be seen, considerable deflections of the parameter K occur fromapproximately the 9th power stroke. This signifies an increased unevenrunning of the internal combustion engine, which is to be attributed tounfavorable operating conditions such as, for example, during theheating of a catalytic converter. In this case the extreme decline ofthe parameter K in the 11th and 12th power stroke is due to misfires VA.The misfires VA can be detected without difficulty by a comparison withthe threshold value S. However, if the threshold value S remainsunchanged, this leads to misfires being detected even though no misfireshave actually occurred. The threshold value S must therefore be adjustedaccordingly for operation of the internal combustion engine withincreased uneven running, which is indicated by the straight linedesignated by S′.

The method according to the invention permits an automatic adjustment ofthe threshold value S to the even running of the internal combustionengine. An embodiment of this method will now be explained in furtherdetail with reference to the flowchart shown in FIG. 2.

The parameter K is continually determined by means of a conventionalmisfire detection method (step 1). Following this, the variance of theparameter K, in other words the changes in K, is determined within apredefined variation range. A predefined time interval or a predefinednumber of power strokes Z can be selected for example as a variationrange. If the variance remains substantially unchanged, the programreturns to step 1.

If, however, the variance of the parameter K changes, this change isanalyzed in a step 3. In particular a check is made to determine inwhich direction and to what extent the variance of the parameter K haschanged.

Depending on the result of this analysis, the threshold value S is thenincreased or reduced (step 4). In the exemplary embodiment shown, thisincrease or reduction in the threshold value S is performed cyclicallyand incrementally.

In this way the threshold value S can be adjusted automatically andcontinuously to the even running or uneven running of the internalcombustion engine during the entire lifetime of the internal combustionengine. It is thus ensured that error-free misfire detection is possibleeven during unfavorable operating conditions of the internal combustionengine.

A further aspect of the method according to the invention is explainedwith reference to the flowchart shown in FIG. 3. In this case use ismade of the method for checking the even running regulation of aninternal combustion engine, as explained with reference to FIG. 2. Asalready explained in the introduction to the description, methods forregulating the even running of an internal combustion engine are knownin the prior art (e.g. DE 197 41 965 C1). In said methods, differenttorque contributions of the individual cylinders are adjusted to matchone another by intervention in the firing and/or fuel injection in orderto improve the even running of the internal combustion engine. Themethod illustrated in the flowchart in FIG. 3 is used for checking theresult of an even running regulation of this kind.

The starting point is once again a conventional misfire detection methodwherein the parameter K is continually determined (step 5). In a step 6,a conventional even running regulation method is now activated in orderto improve the even running of the internal combustion engine.

During this time the method for adjusting the threshold value S (steps 7and 8), as explained with reference to the flowchart shown in FIG. 2,continues to execute. In a step 9 a check is made to determine whetherthe even running regulation has been completed. This can be the case,for example, after a predefined time interval, a predefined number ofpower strokes or upon reaching predefined limits for specific operatingparameters.

As a result of the even running regulation, the variance of theparameter K is reduced. This leads in turn to a corresponding adjustmentof the threshold value S. In step 10 a check is made to determinewhether this threshold value adjustment has been completed.

Following termination of the threshold value adjustment, a check is madewith the aid of the continually performed misfire detection to determinewhether misfires are still occurring in one (or more) cylinders (step11). If no further misfires occur, the program returns to its startingpoint. If, however, it is shown that misfires are continuing to occur inone (or more) cylinders, this is a sign that the combustion in thecylinder in question is defective due to a persistent malfunction. Thisleads to a confirmation of the misfire detection (step 12) and acorresponding input into the operation control system (step 13).

The described method thus allows a higher level of reliability in thedetection of a real malfunction with regard to the combustion in one ormore cylinders.

1. A method for detecting misfires in an internal combustion engine, comprising: operating the internal combustion engine; continually determining a parameter that depends on an acceleration of the internal combustion engine by a monitoring and analysis method; and detecting the misfire based on a comparison of the parameter with a threshold value, wherein a variance in the parameter is determined and used to adjust the threshold value to take account of changes in the even running of the internal combustion engine.
 2. The method as claimed in claim 1, wherein the threshold value is increased if there is a reduction in the even running of the engine and reduced if there is an increase in the even running of the engine.
 3. The method as claimed in claim 1, wherein the adjustment of the threshold value is constantly repeated cyclically during operation of the internal combustion engine.
 4. The method as claimed in claim 1, wherein the method is used during the calibration of the internal combustion engine.
 5. The method as claimed in claim 1, wherein a predefined time interval or a predefined number of power strokes is used in each case as the variation range for the variance of the parameter.
 6. The method as claimed in claim 1, wherein the method is performed on an engine cylinder-specific basis.
 7. The method as claimed in claim 1, wherein an even running regulation method is used to correct the combustion in the cylinders of the internal combustion engine to increase the even running of the engine and the variance of the parameter is used to check the result of the even running regulation.
 8. The method as claimed in claim 7, wherein if after the even running regulation has been performed and the adjustment of the threshold value has been completed, misfires continue to occur in a cylinder, the combustion of the cylinder is detected as defective. 